The present invention concerns a method for fabricating a thin film of polymer, particularly a semiconducting thin film of polymer with an improved structural order, wherein the thin film is formed by deposition of a polymer material onto a solid substrate material, from a polymer solution formed by means of a solvent.
It is well-known to persons skilled in the art that liquid phase epitaxy offers a fast and simple method for fabricating high purity films and semiconductor compounds and alloys. In principle liquid phase epitaxy concerns growing a crystalline layer of a material on a crystalline substrate. The layer is grown from a solution or melt of material by the substrate being dipped in or brought into contact with the solution or melt. For liquid phase epitaxy there is in Joy George, xe2x80x9cPreparation of thin filmsxe2x80x9d, Section 7.2 pp. 311-312 disclosed a so-called dip-coating method where a vertical system is used (see FIG. 7.3), as proposed by H. Ruprecht, xe2x80x9cGallium Arsenidexe2x80x9d Institute of Physics conference, Vol.3, p. 57, (J. Frankel Editor, Bristol Institute of Physics (1967)). The melt or solution is here provided in a crucible in a vertical column and the substrate immersed into the solution and withdrawn vertically from it. Both immersion and withdrawal take place under specific temperature conditions. The vertical column is open in the sense that it during the process is flushed by a reducing gas or a gas with a reduction agent in order to prevent oxidation of the thin film.
In the present invention the subject is, however, not liquid phase epitaxy, but an ordinary deposition of a thin film from a polymer solution and it is therefore in no way evident that the system as the above-mentioned for liquid phase epitaxy, straightforwardly may be applied in the method according to the present invention.
The adsorption of polymer materials from a solution onto a solid substrate has been studied both theoretically and experimentally and in that connection reference shall be made to the following publications, viz. A. Silberberg, Discuss. Faraday Soc., 59: 203 (1976); W. H. Grant, L. E. Smith and R. R. Stromberg, Discuss. Faraday Soc., 50: 209 (1976); Y. Cohen and S. Reich, xe2x80x9cOrdering Phenomena in Thin Polystyrene Filmsxe2x80x9d, Journal of Polymer Science: Polymer Physics Edition, 19: 599-608 (1981); and S. E. Rickert, C. M. Balik and A. J. Hoopfinger, xe2x80x9cOrganization and Assembly of Macromolecules on Inorganic Substratesxe2x80x9d, Advances in Colloid and Interface Science, 11: 149-191 (1979).
Silberberg (1976) and Grant and al. (1976) showed that adsorption of polystyrene on inorganic surfaces takes place in a two-step process. The first step consists in a rapid and simple segmental attachment. The second step consists in a molecular rearrangement, something which takes place very slowly. Completely adsorbed polymers can be regarded as many molecular segments packing on the surface.
FIGS. 1a-c show possible arrangements of polymer chains and particularly FIG. 1a shows an amorphous arrangement, FIG. 1b a semicrystalline arrangement and FIG. 1c a crystalline arrangement. According to Grant and al. (1976) these structures are present on the substrate due to a molecular reorientation after the adsorption. In order to improve the chain ordering it is important that the solvent which was used for forming the polymer solution is retained long enough to achieve molecular reorientation in the polymer material. In prior art solvent casting is a widely used method for fabricating photoresist thin films and this method is also used for fabricating organic thin-film devices of polymer. In the fabrication of organic thin-film transistors based on poly(3-hexyl thiophene) (P3HT), the active layer is for instance usually spin-coated from a solution of P3HT. During the spin coating the ordering of the polymer chain is, however, no longer possible because the solvent is not retained long enough to achieve chain ordering. Another disadvantage with solvent casting is that radial defects in the pattern very easily arise unless carefully selected spinning velocities are used. In Mauro Gibson and Jaime Frejlich. xe2x80x9cDip-coating method for fabricating thin photoresist filmsxe2x80x9d, Thin Solid Films, 128: 161-170 (1985) an investigation is presented of a dip-coating system for fabricating photoresist thin film and it was shown that it was possible to fabricate homogenous photoresist films with good repeatability. The dip-coating system according to Gibson and al. is, however, an xe2x80x9copenxe2x80x9d system, the container with the solution and the solvent being open to the atmosphere and possibly evaporated solvent will, of course, be leaking out.
The object of the present invention is hence to provide a method for fabricating thin films of polymer material on a solid substrate material such that the disadvantage with prior art and particular with the use of so-called xe2x80x9copenxe2x80x9d dip-coating systems are avoided. Further it is an object of the invention to be able to fabricate a very thin and homogenous thin film. Finally it is also an object to retain the solvent sufficiently long to obtain a chain ordering in the deposited thin film.
The above-mentioned and other objects and advantages are achieved with a method according to the invention which is characterized by providing the polymer solution in a portion of a closed container such that a free volume is left in the container above the portion with said polymer solution, immersing the substrate material in said polymer solution, withdrawing said substrate material with a thin film deposited thereupon from said polymer solution with a withdrawal speed being selected dependent on the concentration of said polymer solution, until said substrate material is located in said free volume at a certain distance above said polymer solution, and keeping said substrate material in a vertical position in said free volume, while the solvent evaporates, whereafter said substrate material with said thin film is removed from said container for further drying in a vacuum oven.
According to the invention the thickness of the deposited thin film for a given concentration of the polymer solution is advantageously regulated by regulating the withdrawal speed, the thickness increasing with the concentration of the polymer solution and increasing withdrawal speed.
It is according to the invention also advantageous that the substrate is kept in the vertical position at a distance of 1 cm above the surface of the polymer solution.
Finally it is according to the invention advantageous that the substrate is kept in the vertical position for a time period, the duration of which being selected dependent on the thickness of the thin film and the solvent concentration in the free volume. Preferably the substrate with the deposited thin film is kept in the vertical position for 20-30 min.